1. Field of the Invention
The present invention relates to a method in which an injection molded preform is transferred to a blow mold and is stretch blow molded to a thin-wall hollow molded article such as a bottle or a wide-mouth container and the like.
2. Description of the Related Art
Injection stretch blow molding is known in which material resins such as polyethylene terephthalate (PET), polypropylene (PP), polycarbonate (PC), polyethylene (PE) and the like are carried as a melt by an injection apparatus and injected to fill a cavity of an injection mold. A closed-end preform, in which a body part and a neck part are integrally molded by the injection mold, is transferred to a blow mold by releasing from the injection mold while in a high temperature state. In the blow mold, by means of elongation of a stretch rod and air blowing, the perform is formed into a packaging container such as a bottle having a thin-wall body part or a wide-mouth container and the like.
Among injection stretch blow molding methods, a method has been disclosed in which a preform is released from an injection cavity mold and an injection core, and is transferred to a blow mold while a neck part of the preform is supported by a neck mold (U.S. Pat. No. 5,364,585). Also, with the object of shortening the injection molding time, a method has been disclosed in which a preform is released from an injection cavity mold while being supported by an injection core and neck mold, and cooling of the preform is continued while simultaneously transferring the perform to a blow mold (U.S. Pat. No. 5,589,130).
The above mentioned conventional method, in which an injection molded preform is stretch blow molded immediately while in a high temperature state, is called a hot parison method or one-stage method, as contrasted with a cold parison method, in which an injection molded solid preform is heated to the mold temperature at stretch blow molding. With the cold parison method, since a body part and a bottom part except a neck part are heated by external heating, the difference between high and low temperatures in the wall part during stretch blow molding is small and a large difference does not occur in the degree of molecular orientation generated by the stretch, so that a bottle having a high top load value can be obtained.
In the case of the hot parison method, since the preform is in a high temperature state from being injection molded, the temperature distribution in a cross section of the body wall has a mountain shape, in which the temperature of the central part is high compared with the cooled inner and outer sides, and the temperature difference is large. The temperature difference becomes smaller during stretch blow molding, but is not eliminated completely. The temperature difference influences the compression strength as a result of the difference in degree of molecular orientation by the stretch, so that the top load value is considered incapable of being as much as the value by the cold parison method.
As a means to shorten the cooling time after injection, in the case of the conventional method in which the preform is released from the injection cavity mold together with the injection core, and core cooling is continued even after the mold-release by cooling at the inner side by the injection core, a temperature difference occurs between the inner and outer sides, so that the temperature distribution changes from a mountain shape to a slope shape with the temperature gradient having a lower inner side temperature. But along with the cooling time, the temperature difference between the inner and outer sides grows larger and the temperature gradient is steep. Also, since a thick inner side skin layer is formed, stretch blow molding is difficult with the steep gradient temperature distribution, so that a bottle with a good molding state can not be obtained.
In the slope-shaped temperature distribution in which the temperature is lower toward the inner side of the body wall, since there is no protruding high temperature as with the mountain shape, it is difficult to generate a multiple layer formation from the difference of degree of orientation caused by stretching. Accordingly, if by some measure the inner and outer side temperatures can be made nearer, to make the temperature distribution a gentle gradient, such as in the cold parison method, then even considering the hot parison method, a bottle having a high top load value can be molded.
As for polyethylene terephthalate, for which the whitening phenomenon of the preform is not generated if the outer surface temperature is 100° C. or more, a greater improvement can be provided if the stretch blow molding can be conducted at a higher mold temperature than described in the conventional method.